1. Technical Field
The present invention relates to a metal cathode used in electron-beam devices, and more particularly, to a thermoelectron-emissive metal cathode and an indirectly heated cathode assembly employing the thermoelectron-emissive metal cathode that can be applied to electron-beam devices.
2. Description of the Background Art
As a conventional cathode used in an electron tube, oxide cathodes have been widely used. An oxide cathode includes an electron emissive material layer on a metal base containing nickel (Ni) as a main component and a trace reducing agent such as silicon (Si) or magnesium (Mg). The electron emissive material layer is formed of a carbonate oxide of alkaline earth metals containing barium (Ba) as a main component, but preferably, an oxide of a ternary carbonate of (Ba, Sr, Ca)CO3 or a binary carbonate of (Ba, Sr)CO3. Such oxide cathodes are operable at a relatively low temperature of 700-800xc2x0 C. (Celsius) due to their low work function, but have a limited electron emission capability with a current density less than 1 A/cm2 (Amperes per centimeter square). Furthermore, since the oxide cathode is formed of a semiconductor and has high electrical resistance, degradation of the cathode is resulted from evaporation or melting of the material caused by self-heating by Joule heat as the current density is increased. Also, an intermediate resistance layer is formed between the metal base and the oxide layer with increased use, thereby shortening the life span of the cathode.
The oxide cathode is fragile and has weak adhesion to the metal base. Thus, adopting this type of cathode can reduce the life of a cathode-ray tube (CRT). As an example, a costly color picture tube that needs three oxide cathodes may fail entirely if only one of the oxide cathodes is damaged.
For this reason, approaches to using a high-performance metal cathode, which is free from the drawback of the oxide cathode, in a CRT are increasing. In addition, in order to keep in step with the recent need for a Braun tube with a larger-sized screen, longer lifetime, high-definition, and high-luminance, there is a need for a cathode having a longer lifetime at high current density.
For example, a lanthanum hexaboride (LaB6) based metal cathode, which has been developed to meet the above requirement, is more durable and has greater electron emission capability than oxide cathodes. The monocrystaline cathode of lanthanum hexaboride has a high current density of about 10 A/cm2. However, the short life span of the lanthanum hexaboride based metal cathode limits its application to only a vacuum electron device with a replaceable cathode unit. The short life span of the lanthanum hexaboride based metal cathode is due to a high reactivity to the constituent material of a heater. Lanthanum hexaboride changes into a weakly bound compound by contact with tungsten, which is common as a material of a heater.
U.S. Pat. No. 4,137,476 issued to Ishii et al. for Thermionic Cathode, discloses a cathode with a barrier layer between a lanthanum hexaboride cathode and the body of a heater for blocking possible reaction between the cathode and the heater. However, the cathode has a considerably high manufacturing cost and a poor life extension effect. Research into and development of a cathode based on secondary electron emission and an impregnated cathode are being conducted, but they still have the problems of short lifetime and high production cost.
The conventional metal cathode causes many problems during operation due to its high operating temperature, such as a current leakage between the heater and cathode or a heater disconnection. USSR Patent No. 1975520 issued to Petrovich et al. for Cathode of Electronic Device, discloses a method of adding an alkali metal in forming a cathode of a metal alloy having a platinum group metal and alkaline earth metal so as to lower the operating temperature and increase a secondary emission coefficient. USSR Patent No. 1975520 discloses in particular cathodes that are made of alloys of Nixe2x80x94Mgxe2x80x94Li, Nixe2x80x94Srxe2x80x94Li, or Nixe2x80x94Caxe2x80x94Li. The working temperatures of cathodes made specifically of alloys of Nixe2x80x94Mgxe2x80x94Li, Nixe2x80x94Srxe2x80x94Li, or Nixe2x80x94Caxe2x80x94Li were lowered, however, the problem of the high working temperature of cathodes made of alloys of Ptxe2x80x94Ba or Pdxe2x80x94Ba were in reality not solved. The cathodes made of alloys of Ptxe2x80x94Ba or Pdxe2x80x94Ba still had high operating temperatures of up to 1200xc2x0 C. (Celsius), thereby causing a problem of heater disconnection and current leakage when applied to a Braun tube, as described above.
Furthermore, the manufacture of a conventional CRT involves an aging process at a higher temperature, i.e., about 1300xc2x0 C. (Celsius), than the operating temperature of cathodes, i.e., 1100-1200xc2x0 C. (Celsius) to warm up the cathodes. Such an aging process creates a serious risk of damage such as current leakage and heater disconnection.
Although metal cathodes such as those described above can be used in a CRT, thermal distortion of neighboring electrodes, particularly, the G1 electrode, caused by a metal cathode working at a high temperature results in increased stabilization time, thereby limiting practical uses.
It is therefore an object of the present invention to provide a thermoelectron-emissive metal cathode for electron-beam devices such as a Braun tube and picture tube, with a low operating temperature, good electron emission capability, and an extended life span at high current density.
It is another object of the present invention to provide an indirectly heated cathode assembly employing the above thermoelectron-emissive metal cathode.
To achieve the above and other objects of the present invention, there is provided a metal cathode for an electron-beam device, the metal cathode including an electron-emitter formed of a quaternary alloy including 0.1-20% by weight barium (Ba), 0.1-20% by weight a metallic mobilizer facilitating Ba diffusion, 0.01-30% by weight a metal with a difference in atomic radius of at least 0.4 xc3x85 (Angstrom) from the atomic radius of platinum (Pt) or palladium (Pd), and a balance of at least one of Pt and Pd.
In the metal cathode according to the present invention, it is preferable that the metallic mobilizer facilitating Ba diffusion is at least one member selected from the group including molybdenum (Mo), hafnium (Hf), zirconium (Zr), and thorium (Th).
In the metal cathode according to the present invention, it is preferable that the metal with a difference in atomic radius of at least 0.4 xc3x85(Angstrom) from the atomic radius of platinum (Pt) or palladium (Pd) is at least one member selected from the group including calcium (Ca), strontium (Sr), and cerium (Ce). It is also preferable that the metal with a difference in atomic radius of at least 0.4 xc3x85 (Angstrom) from the atomic radius of platinum (Pt) or palladium (Pd) is an alloy of cerium (Ce) and iridium (Ir). The alloy of cerium and iridium is preferably Ir5Ce.
There is also provided a metal cathode for an electron-beam device, the metal cathode including an electron-emitter formed of a quaternary alloy including 0.1-20% by weight barium (Ba), 0.1-20% by weight a metallic mobilizer facilitating Ba diffusion, 0.01-30% by weight a metal with a difference in atomic radius of at least 0.4 xc3x85 (Angstrom) from the atomic radius of platinum (Pt) or palladium (Pd), and a balance of at least one of Pt and Pd, and a layer of a material having a larger work function than above quaternary alloy coated on the electron-emitter. It is preferable that the material having a larger work function than the quaternary alloy is one of iridium (Ir) and an alloy of osmium (Os) and ruthenium (Ru). It is preferable that the coated layer has a thickness of 500-30,000 xc3x85 (Angstrom). It is preferable that the alloy of Os and Ru contains 1-10% by weight Ru.
In the metal cathode according to the present invention, it is preferable that the metallic mobilizer facilitating Ba diffusion is at least one selected from the group including molybdenum (Mo), hafnium (Hf), zirconium (Zr), and thorium (Th).
In the metal cathode according to the present invention, it is preferable that the metal with a difference in atomic radius of at least 0.4 xc3x85 (Angstrom) from the atomic radius of platinum (Pt) or palladium (Pd) is at least one selected from the group including calcium (Ca), strontium (Sr), and cerium (Ce). It is also preferable that the metal with a difference in atomic radius of at least 0.4 xc3x85 (Angstrom) from the atomic radius of platinum (Pt) or palladium (Pd) is an alloy of cerium (Ce) and iridium (Ir), and more preferably Ir5Ce.
An indirectly heated cathode assembly employing the metal cathode described above is also preferable.